The present invention is directed to a circuit for the subscriber termination in an integrated services digital network (ISDN) in which text, pictures and data are communicated (transmitted and switched) in addition to speech in common telecommunication channels having, for example, a bit rate of 64 kbit/s per transmission direction. Subscribers are enabled for use of the various telecommunications services by means of a common subscriber termination to which terminal equipment for telephony, text communication, picture communication and data communication can be connected in a variety of configurations. The ISDN subscriber termination must exhibit basic properties suitable for this purpose, particularly with respect to the bit rate, the signaling possibilities, and the range.
A so-called basic access having a channel structure designated B+B+D (per transmission direction) has evolved in this context in the international discussion and standardization of this field at the CEPT and the CCITT, wherein B designates a 64 kbit/s channel which can transmit digitally encoded information of different types such as voice, text, facsimile, and data simultaneously, alternatively from connection to connection, or within one connection or in fixed allocation, and wherein D is a 16 kbit/s channel in which signaling characters (with priority) and telemetry signals and slow packet oriented data can be transmitted in multiplex.
The subscriber termination has an ISDN line interface, the so-called U-interface at the end of the subscriber line and an ISDN terminal interface, the so-called subscriber or S-interface, which is connected to the U-interface through a so-called network termination for functions within the level 1 and, under given conditions, within levels 2 and 3 of the ISO function level model. It is assumed that the S-interface has multi-point capability, that is, multiple device configurations can be formulated by means of branching at the S-interface, a plurality of ISDN terminals with S-interface being arranged therein in a bus, star or ring structure as described, for example, in "ZPF" 1982, volume 9, pages 24-31; Fernmeldetechnik 22 (1982) 2, pages 67-79; Fernmeldetechnik 23 (1983) 1, pages 6-9 and 9-14.
In an S-interface bus configuration, all connected terminals can access the D-channel (signaling channel) independently of one another, an allocation of a B-channel being effected thereover by the exchange equipment. Non-interfering access to the S-bus is controlled by the connected terminal in the D-channel itself. All bits proceeding from the terminals through the D-channel to the line termination which are to be forwarded from the line termination to the switching center are subjected to an AND operation for all simultaneously appearing bits and are thereafter fed back to the terminals through the S-interface in a separate D-echo channel in such a manner that 0 bits dominate over 1 bits, that is, when different terminals simultaneously transmit 0 bits and 1 bits in the D-channel, a 0 bit is always returned in the D-echo channel, so that a terminal which has transmitted a 1 bit perceives this deviation and discontinues its accessing attempt.
In the bus configuration, the S-interface enables connection of up to eight terminals given a bus length of about 150 meters, the range being about 1 kilometer in an individual device configuration having only one terminal or in a star configuration wherein each terminal is connected to a network termination through a separate line.
When a number of individual terminals are to be connected to a common ISDN network termination through separate S-interfaces, a problem is the manner by which the individual interface-associated D-channels in the network termination unit are to be united with the common D-channel proceeding to the switching control. It is an object of the present invention to provide a circuit for solving this problem.
The above objects are inventively achieved in a circuit for a subscriber termination in an integrated services digital network wherein monofunctional or multifunctional terminals comprising an S-interface (which may be obtained under given conditions through a matching unit) are connected through a network termination to the subscriber line which forms an ISDN basic access. The B-channels proceed through the subscriber line between the ISDN network termination and a switching center and are respectively allocated to a terminal controlled by the switching center together with the D-channel combined therewith in multiplex, or fanned from multiplex in the network termination. The individual terminal-associated or bus-associated D-channels in the transmission direction toward the switching control are conducted through an AND gate to the common D-channel, which continues to the switching center. The output of the AND gate is also connected through a common D-echo channel to the individual terminal-associated or bus-associated D-echo channels of the opposite transmission direction.
The circuit disclosed and claimed herein as described above, which makes use of level 1 access protocol for accessing an S-bus in modified form for combining the D-channels of a number of S-interfaces individually conducted to the network termination unit with the common D-channel, and which can be employed both for S-terminations of the same subscriber as well as to S-termination of different subscribers, offers the advantage of an uncomplicated realization of individual connection of a number of individual terminals to the common ISDN network termination unit by the terminal's own ISDN S-interface. Additionally the circuit provides greater flexibility in the configuration of ISDN multi-device terminations because the circuit permits connections of ISDN S-interface busses respectively combining a number of terminals in a common ISDN network termination.